It occurs to me that 100 is 2% more than 7×7×2, so you could reasonably approximate 7× (if you didn't know it) as follows. Since 1dB is about 23%, 2% is about 0.09dB, so 98× is about 19.91dB, 49× is about 16.90dB, and so 7× has to be half that: 8.45dB. And so it is.